As a drill for drilling holes, there are for example those in which an inner insert and an outer insert are detachably attached to the tip end of a holder so that their respective rotation loci cross each other. Among others, those in which the inner insert and the outer insert have the same shape are frequently used from the viewpoint of excellent economic performance. That is, the drills in which a plurality of cutting inserts for a drill of similar type (hereinafter referred to as “insert” in some cases) are respectively detachably attached to the inner side and the outer side at the tip end of the holder are frequently used.
The inserts used for this type of drill include an inner cutting edge and an outer cutting edge. The inner cutting edge is the cutting edge for mainly cutting (machining) an inner portion of a bottom face of a hole when it is used as the inner insert. The outer cutting edge is the cutting edge for mainly cutting an outer portion of a bottom face of a hole when it is used as the outer insert.
For example, in the insert described in Japanese Unexamined Patent Application Publication No. 10-29108, the inner cutting edge and the outer cutting edge are formed at the intersection between the upper face and the side face. A clearance angle for avoiding interference with a work material is provided on each of the side faces corresponding to these cutting edges. A corner portion located between both of these cutting edges is formed in a round shape. The same clearance angle as that on the side faces corresponding to these cutting edges is provided on the side face corresponding to the corner portion (refer to FIG. 6(b) in the above publication).
On the other hand, the holder used for this type of drill has, at its tip end, an inner peripheral insert pocket for attaching the inner insert, and an outer peripheral insert pocket for attaching the outer insert (refer to, for example, the above publication).
The outer peripheral insert pocket is openedly formed at the outer periphery of the holder. On the other hand, the inner peripheral insert pocket has a wall face which is located toward the outer periphery of the holder and opposed to the side face of the inner insert. An outer wall exists between the wall face and the outer peripheral face of the holder (refer to FIGS. 2 and 3 of the above publication).
The thickness of the outer wall becomes minimum between the wall face opposed to the side face of the corner portion located on the side not to be involved in cutting in the insert, and the outer peripheral face of the holder. Therefore, there has been the problem that the strength of the portion of the holder where the thickness of the outer wall becomes minimum is deteriorated, and the holder is susceptible to breakage during machining.
Additionally, the outer wall does not exist in the outer peripheral insert pocket formed openedly in the outer periphery, and the portion of the holder where the outer peripheral insert pocket is formed has low strength. Hence, there has also been the problem that the holder is liable to be broken from the portion of the holder during machining.